The concave surface of a spectacle lens is formed in the shape of a spherical surface, a rotation-symmetric non-spherical surface, a toric surface, a progressive surface, or a curved surface synthesized therefrom, or the like. When the surface shape is processed by cutting or the like, the surface is mirror-polished to be an optical surface. The mirror polishing of a simple curved surface such as a spherical surface and a toric surface is conducted by fit polishing by use of a polishing dish of a rigid material called Oscar system. The mirror polishing method using the polishing dish is a method of transferring the surface shape of the polishing dish to the work (the object to be polished). Therefore, a number of processing dishes corresponding to the number of surface shapes according to the lens prescriptions, for example, thousands of processing dishes are needed.
The polishing dishes cannot be used for polishing of complicated, so-called free curved surfaces such as progressive surfaces other than these curved surfaces. Therefore, elastic polishing bodies are generally used.
For example, there has been known a partial polishing method in which a small dome-shaped elastic polishing body capable of making contact with a part of the work surface (surface to be polished of the work) is used. In the partial polishing method, the maximum curvature is determined from the shape of the work surface, a dome-shaped elastic polishing body having a curvature greater than the maximum curvature is selected, the elastic polishing body is brought into contact with a part of the work surface while being rotated, and the elastic polishing body is moved scanningly over the entire part of the work surface, whereby the entire part of the work surface is polished. Since the elastic polishing body can follow up to the shape of the work surface through deformation thereof, most of the curved surfaces can be polished by use of a single kind of elastic polishing body.
In addition, a polishing method using a balloon-type polishing body has also been known. In this method, a pressurized gas is fed into the inside of the balloon-type polishing body to inflate the balloon-type polishing body by the internal pressure. Then, the entire work surface is polished by bringing the balloon-type polishing body into contact therewith. The curvature of the balloon-type polishing body is changed to match the curved surface shape of the work. The curvature is changed by varying the internal pressure of the balloon-type polishing body, at the time of polishing. Since the balloon-type polishing body can thus follow up to the concave curved surface to be polished, it is possible to cope with a multiplicity of work surfaces by use of a single kind of balloon-type polishing body.
However, in the partial polishing method, partial polishing polishes the entire part of the work surface, which leads to a longer polishing time and a higher production cost. Besides, when it is intended to cope with various curved surfaces by controlling the shape of the elastic polishing body through regulation of the polishing pressure, pressure deficiency and/or pressure excess would be partially generated in the work surface. Therefore, polishing the entire part of the work surface at a uniform polishing pressure is difficult. Thus, it is difficult to achieve uniform polishing.
In addition, the polishing method using the balloon-type polishing body requires a shorter polishing time, because polishing is conducted by bringing the balloon-type polishing body into contact with the entire part of the work surface. However, a rise in the internal pressure may result in that the flexibility is lost, the performance of following up to the shape of the work surface is lowered, and nonuniform polishing is generated. On the other hand, when the internal pressure is lowered, the pressure of contact of the balloon-type polishing body with the work surface is lowered. The sufficient polishing may be impossible. Accordingly, the change of the curvature of the balloon-type polishing body through regulation of the internal pressure is attended by a narrow allowable range and is therefore not so effective.
The present invention has been made in consideration of the above-mentioned problems. Accordingly, it is an object of the present invention to provide a polishing method favorably following up the curvature of a concave work surface and mirror-polishing the work surface uniformly and rapidly.
It is another object of the present invention to provide a polishing device with which the polishing method of the invention can be realized.